Switch cmd/ to use constructor functions. (#36962)

This is a step towards potentially splitting command groups into their
own folders to clean up `cmd/` as one folder for all cli commands.
Returning fresh command instances will also aid in adding tests as you
don't need to concern yourself with the whole command tree being one
mutable variable.

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

cmd/hook.go

@@ -28,23 +28,24 @@ const (
 	hookBatchSize = 500
 )
 
-var (
-	// CmdHook represents the available hooks sub-command.
-	CmdHook = &cli.Command{
+func newHookCommand() *cli.Command {
+	return &cli.Command{
 		Name:        "hook",
 		Usage:       "(internal) Should only be called by Git",
 		Hidden:      true, // internal commands shouldn't be visible
 		Description: "Delegate commands to corresponding Git hooks",
 		Before:      PrepareConsoleLoggerLevel(log.FATAL),
 		Commands: []*cli.Command{
-			subcmdHookPreReceive,
-			subcmdHookUpdate,
-			subcmdHookPostReceive,
-			subcmdHookProcReceive,
+			newHookPreReceiveCommand(),
+			newHookUpdateCommand(),
+			newHookPostReceiveCommand(),
+			newHookProcReceiveCommand(),
 		},
 	}
+}
 
-	subcmdHookPreReceive = &cli.Command{
+func newHookPreReceiveCommand() *cli.Command {
+	return &cli.Command{
 		Name:        "pre-receive",
 		Usage:       "Delegate pre-receive Git hook",
 		Description: "This command should only be called by Git",
@@ -55,7 +56,10 @@ var (
 			},
 		},
 	}
-	subcmdHookUpdate = &cli.Command{
+}
+
+func newHookUpdateCommand() *cli.Command {
+	return &cli.Command{
 		Name:        "update",
 		Usage:       "Delegate update Git hook",
 		Description: "This command should only be called by Git",
@@ -66,7 +70,10 @@ var (
 			},
 		},
 	}
-	subcmdHookPostReceive = &cli.Command{
+}
+
+func newHookPostReceiveCommand() *cli.Command {
+	return &cli.Command{
 		Name:        "post-receive",
 		Usage:       "Delegate post-receive Git hook",
 		Description: "This command should only be called by Git",
@@ -77,8 +84,11 @@ var (
 			},
 		},
 	}
-	// Note: new hook since git 2.29
-	subcmdHookProcReceive = &cli.Command{
+}
+
+// Note: new hook since git 2.29
+func newHookProcReceiveCommand() *cli.Command {
+	return &cli.Command{
 		Name:        "proc-receive",
 		Usage:       "Delegate proc-receive Git hook",
 		Description: "This command should only be called by Git",
@@ -89,7 +99,7 @@ var (
 			},
 		},
 	}
-)
+}
 
 type delayWriter struct {
 	internal io.Writer